Those who drive vehicles on ice, snow or loose gravel frequently have trouble controlling the spin of the vehicle's driven wheels. Such surfaces have a relatively low coefficient of friction which substantially reduces the traction between the driven wheels and the road surface. As a result, even a small amount of torque applied to the driven wheels tends to cause them to spin. If the spin becomes severe, the vehicle can be difficult to control.
As far as the applicants are aware, there is no known technique for accurately estimating the traction characteristics of a surface under a moving vehicle. Such an estimate is difficult, because the surface under a moving vehicle is subject to rapid change. As a result, the traction characteristic must be quickly estimated, and the estimates must be updated at short time intervals in order to assure accurate estimation on a real time basis as the vehicle is being driven.
One paper which refers to a gross estimate of surface adhesion coefficient is entitled "Development of New Control Methods to Improve Response of Throttle Type Traction Control System," by Hiroshi Igata et al., reprinted from Transmission and Driveline Symposium: Components, Gears and CAE (SP-905), bearing the legend "International Congress & Exposition, Detroit, Mich., Feb. 24-28, 1992," available as SAE Technical Paper Series No. 920608. Page 4 of the paper suggests that the adhesion coefficient can be assumed to be at least equal to the acceleration of the vehicle, as measured in terms of G. Page 5 of the paper suggests that the calculated adhesion coefficient should not be revised to a lower value unless wheel slip exceeds a predetermined level.
Experience has shown that a periodically updated estimate of a traction characteristic of a surface in real time provides advantages over an assumption that the adhesion coefficient is equal to the acceleration of the vehicle, as measured in terms of G.
Experience has also shown that an accurate real time estimate of the traction characteristics of the surface under a moving vehicle can be used to help better control wheel spin.